This invention relates to intrusion detection systems and more particularly to a multimode perimeter intrusion detection system.
A current perimeter intrusion detection system capable of reliably detecting intrusions that involve mechanical excitation of a fence is described in U.S. Pat. No. 3,763,482. This system, however, cannot detect an intruder who crosses the fence without touching it, for example, by climbing an adjacent ladder and jumping over the fence. One way to overcome this limitation is to use a second intrusion detection system, such as one having an infrared beam directed along one or both sides of the fence and which activates an alarm when the beam is interrupted. While this technique is effective, it is substantially more costly than a single fence-mounted system and is more time consuming to install and maintain.
Another perimeter intrusion detection system described in U.S. Pat. No. 4,023,155 comprises a continuous electret tape on the wall of a room or on the ground and capable of radiating ultrasonic waves outwardly from the tape. A person moving through the radiation zone reflects a doppler-shifted signal which is detected and activates an alarm. This system is effective for the single purpose of volumetric intrusion detection and heretofore has been limited to this type of protection.
The general problem experienced with single mode intrusion detection systems is limited protection capability and/or limited reliability. As mentioned above, the fence-mounted cable sensor described in U.S. Pat. No. 3,763,482 only detects vibrations transmitted to it through the fence or otherwise; an intruder who vaults over the fence is home free. While this cable sensor also acts as a line microphone to pick up sonic vibrations incident on it as described in U.S. Pat. No. 3,833,897, a quiet vaulting intruder may still defeat the system. Sonic or ultrasonic volumetric systems provide a physically broader or "blanket" type protection zone but are vulnerable to non-intruder type disturbances such as wind, temperature changes, falling branches or leaves, etc. in generating false alarms, thus making them less reliable. The above limitations of single mode systems can be reduced or minimized by redundancy, that is providing two or more separate and different systems but this approach is costly.
This invention is directed to a solution to these problems.